More than half of organic carbon on earth is found in the cell walls of plants. Plant cell walls comprise three main compounds: cellulose, hemicellulose, and lignin. Collectively these compounds are called “lignocellulose,” and they represent a potential source of sugars and other organic molecules for fermentation to ethanol or other high-value products.
The conversion of lignocellulosic biomass to ethanol has become a key feature of emerging energy policies due to the environmentally favorable and sustainable nature of cellulosic ethanol. There are several technologies being developed for cellulose conversion. Of interest here is a method by which lignocellulosic biomass is subjected to a pretreatment that increases its susceptibility to hydrolytic enzymes, followed by enzymatic hydrolysis to sugars and the fermentation of those sugars to ethanol or other high-value organic molecules (e.g. butanol). Common pretreatment methods include dilute acid steam explosion (U.S. Pat. No. 4,461,648), ammonia freeze explosion (AFEX; Holtzapple et al., 1991), and organosolv extraction (U.S. Pat. No. 4,409,032). Hydrolysis and fermentation systems may be either separate (sequential hydrolysis and fermentation; SHF) or coincident (simultaneous saccharification and fermentation; SSF). In all instances, the hemicellulose and cellulose are broken down to sugars that may be fermented, while the lignin becomes separated and may be used either as a solid fuel or as a source for other organic molecules.
The choice of enzymes for conversion of pretreated lignocellulosic biomass to sugars is highly dependent upon the pretreatment method. Dilute acid steam explosion results in significant chemical hydrolysis of the hemicellulose, thereby making enzymes for the conversion of hemicellulose to sugars less relevant to the process. In contrast, AFEX and organosolv extraction both leave hemicellulose and cellulose largely intact. Organosolv extraction, unlike dilute acid steam explosion or AFEX removes a significant portion of the lignin from substrate. In all instances, the primary target for enzymatic hydrolysis is the cellulose, which is converted to sugars using a combination of cellulase enzymes.
There are two principle types of cellulase enzymes: endoglucanases, which cleave glycosidic bonds in the middle of cellulose chains, and in doing so, create new chain ends, and cellobiohydrolases, which cleave short oligosaccharides from the ends of cellulose chains. Glucosidases digest short oligosaccharides into monosaccharides. These three enzyme components thus act synergistically to create an efficient cellulolytic enzyme system. Most cellulases have a similar modular structure, which consists of a catalytic domain, linker peptide and a carbohydrate-binding module (CBM).
Modified cellulase enzymes and methods for modification have been extensively described. For example, variants of Trichoderma reesei Cel7A and Cel6A to improve thermostability have been reported (U.S. Pat. No. 7,375,197; WO 2005/028636; U.S. Publication No. 2007/0173431; U.S. Publication No. 2008/167214; WO 2006/074005; U.S. Publication No. 2006/0205042; U.S. Pat. No. 7,348,168; WO 2008/025164). In particular, substitution of the serine at position 413 in T. reesei Cel6A with a proline, or substitution of the amino acid at the equivalent to position 413 with a proline in other Family 6 cellulases confers increased thermostability (WO 2008/025164). Mutations at the equivalent of positions 103, 136, 186, 365 and 410 within the catalytic domain of T. reesei Cel6A and other Family 6 cellulases have been shown to lead to reduced inhibition by glucose (U.S. Patent Publication No: 2009/0186381). Variants with resistance to proteases and to surfactants for detergent formulations have been created for textile applications (WO 99/01544; WO 94/07998; and U.S. Pat. No. 6,114,296).
The negative effects of lignin on cellulase enzyme systems are well documented. Removal of lignin from hardwood (aspen) was shown to increase sugar yield by enzymatic hydrolysis (Kong et al., 1992). Similarly, removal of lignin from softwood (Douglas fir) was shown to improve enzymatic hydrolysis of the cellulose, an effect attributed to improved accessibility of the enzymes to the cellulose (Mooney et al., 1998). Other groups have demonstrated that cellulases purified from Trichoderma reesei bind to isolated lignin (Chemoglazov et al., 1988) and have speculated on the role of the different binding domains in the enzyme-lignin interaction (Palonen et al., 2004). Binding to lignin and inactivation of Trichoderma reesei cellulases has been observed when lignin is added back to a pure cellulose system (Escoffier et al., 1991). Only in one instance was lignin reported to not have any significant effect on cellulases (Meunier-Goddik and Penner, 1999). Other reports suggest that some hemicellulases may be resistant to, and even activated by, lignin and lignin breakdown products (Kaya et al., 2000). Thus, it is generally recognized that lignin is a serious limitation to enzymatic hydrolysis of cellulose.
CBMs are reportedly involved in lignin binding. For example, removal of the CBM from Trichoderma Cel7A essentially eliminates binding to alkali extracted lignin and to residual lignin prepared by enzyme hydrolysis (Palonen et al., 2004).
Catalytic domains are also reportedly involved in binding lignin. Cel7B from Humicola sp., which does not possess a CBM, is bound extensively by lignin (Berlin et al., 2005b). Similarly Trichoderma Cel5A core, devoid of a CBM, does not bind enzymic lignin and binds alkali extracted lignin to a lesser extent than does the full-length protein (Palonen et al., 2004).
The development of lignin resistant cellulases with preserved cellulose binding affinity and native cellulolytic activity represents a large hurdle in the commercialization of cellulose conversion to soluble sugars including glucose for the production of ethanol and other products. A variety of methods have been suggested to reduce the negative impact of lignin on the cellulase system. Non-specific binding proteins (e.g. bovine serum albumin; BSA) have been shown to block interactions between cellulases and lignin surfaces (Yang and Wyman, 2006; US24185542 A1; US26088922 A1; WO05024037 A2, A3; WO09429474 A1). Other chemical blocking agents and surfactants have been shown to have a similar effect (Tu et al., 2007; U.S. Pat. No. 7,354,743). While it has been proposed to seek out and identify lignin-resistant variants of cellulase enzymes (Berlin et al., 2005a), no successful work in this direction has been previously documented.